One-layer Langmuir-Blodgett Films Research Articles

Dependences of Thermal Behaviors on the Number of Layers in Langmuir−Blodgett Films of 2-Octadecyl-7,7,8,8-tetracyanoquinodimethane. 1. Order−Disorder Transitions in the Films Investigated by Ultraviolet−Visible and Infrared Spectroscopies

Dependences of thermal behaviors on the number of layers in Langmuir−Blodgett (LB) films of 2-octadecyl-7,7,8,8-tetracyanoquinodimethane (octadecyl-TCNQ) have been investigated for both morphological and molecular structural aspects by using ultraviolet−visible (UV−vis) and infrared (IR) spectroscopies. The temperature-dependent morphological changes have been monitored by the base-line shift in the UV−vis spectra of the LB films. The changes in the base-line in the 450−600 nm region suggest that the domains in the one-layer film decompose progressively with temperature even just above room temperature. In contrast, the domains in the seven- and 11-layer films are stable until 80 °C and undergo rather abrupt collapse between 80 and 90 °C. These differences in the thermal behaviors of the domain structure between the one-layer film and the seven- and 11-layer films may be attributed to the differences in the film thickness, the effect of the interaction between the first layer and a CaF2 substrate, and the longitudinal interaction between the domains. The IR spectral measurements for the LB films at elevated temperatures reveal that the order−disorder transition temperature of the one-layer LB film is lower by about 5 °C than that of the 11-layer film, which is close to the chain melting point of the bulk material. Of particular interest is that the domain structure of the one-layer film shows the gradual temperature change but that the molecular orientation and structure in the domains experience the clear order−disorder transition.

Dark-field electron microscopy of Langmuir-Blodgett films of fatty acids and their barium salts

Monolayers of fatty acids and their barium salts on water were transferred onto supporting films of carbon or silicon dioxide by horizontal attachment, down-stroke or up-stroke modes at various stages of surface-pressure-area (π− A) isotherms. Dark-field electron microscope images and electron diffraction patterns were used to determine the structure and molecular ordering of one-layer Langmuir-Blodgett (LB) films without any presurface treatment such as shadow casting and replication. The observations revealed the polycrystalline powder structure of one-layer LB films of stearic acid, arachidic acid and barium arachidate prepared by the horizontal attachment mode at no compression. With monolayer compression the LB films tended to form a structure consisting of growing island films which included a number of scattered small holes. A dark-field electron micrograph and electron diffraction pattern taken with an electron cryomicroscope led us to conclude that the presence of these scattered small holes in the LB films was due neither to the sublimation of LB film molecules in vacuum nor to electron radiation damage. Similar porous structures were found when the monolayers of barium arachidate with flexible alkyl chains of an intermediate length were transferred by the down-stroke mode onto supporting films of carbon or silicon dioxide. Arachidic acid and its barium salt formed LB films of a rather homogeneous structure with a few large holes by the up-stroke mode. Because the cohesive force increased with the length of alkyl chains, the one-layer LB films of melissic acid (C 30) showed a similar porous structure, irrespective of the mode of transfer of the monolayers.